Polyglycolic acid is known to be degraded by microorganisms or enzymes present in the natural world such as soil and sea because it contains aliphatic ester linkages in its molecular chain. In recent years, the disposal of plastic waste has become a great problem with the increase of plastic products. Polyglycolic acid attracts attention as a biodegradable polymer material which scarcely imposes burden on the environment. The polyglycolic acid has intravital absorbability and is also utilized as a medical polymer material for surgical sutures, artificial skins, etc. (U.S. Pat. No. 3,297,033).
Polyglycolic acid can be produced by dehydration polycondensation of glycolic acid, dealcoholization polycondensation of an alkyl glycolate, desalting polycondensation of a glycolic acid salt or the like. Polyglycolic acid can also be produced by a process comprising synthesizing glycolide, which is a bimolecular cyclic ester (also referred to as “cyclic dimer”) of glycolic acid and subjecting the glycolide to ring-opening polymerization. According to the ring-opening polymerization process of glycolide, high-molecular weight polyglycolic acid can be produced with good efficiency.
Since polyglycolic acid is excellent in heat resistance, gas barrier properties, mechanical strength, etc. compared with other biodegradable polymer materials such as aliphatic polyesters, its new uses have been developed as sheets, films, containers, injection-molded products, etc. [Japanese Patent Application Laid-Open No. 10-60136 (U.S. Pat. No. 5,853,639), Japanese Patent Application Laid-Open No. 10-80990 (U.S. Pat. No. 6,245,437), Japanese Patent Application Laid-Open No. 10-138371, and Japanese Patent Application Laid-Open No. 10-337772 (U.S. Pat. Nos. 6,001,439 and 6,159,416)].
However, the production techniques of the polyglycolic acid is not sufficiently established compared with the general-purpose polymer materials, and so its thermal properties are not always suitable for melt processing, stretch processing, etc. The polyglycolic acid is insufficient in melt stability, for example, in that it tends to generate gasses upon its melt processing.
A homopolymer of polyglycolic acid, and copolymer containing a repeating unit derived from polyglycolic acid in a high proportion are crystalline polymers. Such a crystalline polyglycolic acid is high in crystallization temperature Tc2 detected in the course of its cooling from a molten state by means of a differential scanning colorimeter (DSC) and relatively small in a temperature difference (Tm−Tc2) between the melting point Tm and the crystallization temperature Tc2 thereof. A polymer small in this temperature difference generally has a merit, upon injection molding, that the injection cycle thereof can be enhanced attributable to its fast crystallization speed. However, such a polymer is easy to crystallize upon its cooling from a molten state when it is extruded into a sheet, film, fiber or the like and it is difficult to get an amorphous preform, and so it is difficult to provide any transparent formed product.
The crystalline polyglycolic acid is small in a temperature difference (Tc1−Tg) between a crystallization temperature Tc1 detected in the course of heating of its amorphous substance by means of DSC and the glass transition temperature Tg thereof. A polymer small in this temperature difference generally involves a problem that a stretchable temperature range is narrow upon stretching of a sheet, film, fiber or the like formed from such a polymer, or stretch blow molding of the polymer.
Therefore, the melt processing or stretch processing using a conventional crystalline polyglycolic acid has involved a problem that forming conditions such as forming temperature or stretching temperature are limited to narrow ranges.
Specifically, the present inventors produced polyglycolic acid in accordance with the production process disclosed in Example 1 of U.S. Pat. No. 2,668,162 to investigate the thermal properties of this polyglycolic acid by means of DSC. As a result, its melting point Tm was about 222° C., while its crystallization temperature Tc2, which is an exothermic peak temperature attributable to crystallization when cooling it at a cooling rate of 10° C./min from a molten state at 252° C. higher by 30° C. than the melting point, was 192° C. Accordingly, a temperature difference (Tm−Tc2) between the melting point Tm and the crystallization temperature Tc2 of this polyglycolic acid is about 30° C.
The polyglycolic acid was heated to 252° C. and then held by a press cooled with water to 23° C. to produce a cooled sheet. As a result, the crystallization of the polyglycolic acid was observed on the sheet, and no transparent amorphous sheet was able to be obtained. A transparent amorphous sheet (amorphous film) was able to be obtained with difficulty by melting and pressing the polyglycolic acid and then quenching the resulting sheet in ice water kept at about 4° C. Its crystallization temperature Tc1 detected in the course of heating of such an amorphous sheet by means of DSC was measured. As a result, it was about 75° C., and its glass transition temperature was about 40° C. Accordingly, a temperature difference (Tc1−Tg) between the crystallization temperature Tc2 and the glass transition temperature Tg thereof is about 35° C.
Further, polyglycolic acid is not sufficient in melt stability and has a tendency to easily generate gasses upon its melt processing. More specifically, in the conventional polyglycolic acid, a temperature at which the weight loss upon heating reaches 3% is about 300° C. In addition, it has been found that many of additives such as a catalyst deactivator, a nucleating agent, a plasticizer and an antioxidant deteriorate the melt stability of polyglycolic acid. When the melt stability of polyglycolic acid is insufficient, forming or molding conditions such as forming or molding temperature are limited to narrow ranges, and the quality of the resulting formed or molded product is easy to be deteriorated.